Battery and assembly method

ABSTRACT

An example includes a method including forming a battery electrode by disposing an active material coating onto a silicon substrate, assembling the battery electrode into a stack of battery electrodes, the battery electrode separated from other battery electrodes by a separator, disposing the stack in a housing, filling the interior space with electrolyte, and sealing the housing to resist the flow of electrolyte from the interior space.

CLAIM OF PRIORITY

This patent application claims the benefit of priority, under 35 U.S.C.Section 119(e), to Borden U.S. Provisional Patent Application Ser. No.61/484,854, entitled “BATTERY AND ASSEMBLY METHOD,” filed on May 11,2011 and Borden U.S. Provisional Patent Application Ser. No. 61/525,068,entitled “BATTERY AND ASSEMBLY METHOD,” filed on Aug. 18, 2011, each ofwhich is hereby incorporated herein by reference in its entirety.

BACKGROUND

Battery technology, such as for electric vehicles and renewable energyapplications, is an area of intense research and development. Work hasfocused on a number of technologies, with the most mature and successfulones being lithium-ion and lead-acid batteries. Despite this work, costremains a central concern. Lithium ion, with its energy density, isattractive, but car-makers can pay $1,000/kW·hr or more for alithium-ion power source. Costs remain high due to complex control andcooling systems in addition to electronics used to improve safety. Thiscost is at least six times the United States Advanced Battery Consortium(USABC) year 2020 target of $150/kW·hr. Contrast this with contemporarylead-acid batteries (lead-acid batteries), which can have a cost ofaround $150/kW·hr for renewable energy storage, but their limited energydensity, cycle life, and efficiency in many cases discourages their use.

SUMMARY

Examples described below can improve upon contemporary batteries byproviding a lead-acid battery formed of one or more very thin planarbattery electrodes (e.g., less than 1.0 millimeter) having active mass(e.g., lead or a compound thereof) disposed on a very thin siliconsubstrate (e.g., less than 0.5 millimeters thick). Examples provide animproved battery that is less expensive and that performs better thanother approaches. Because reliability and support infrastructure isimportant to widespread adoption, examples can employ technologies basedon proven batteries chemistries, such as lead-acid. A plurality of theseelectrodes can be stacked together and packaged to provide a lead-acidthat performs better than contemporary lead-acid batteries, such as byavoiding unbalanced ion depletion that can lead to nonreactive leadmaterial. Examples of these batteries, and methods of making and usingthem, are described herein.

This summary is intended to provide an overview of subject matter of thepresent patent application. It is not intended to provide an exclusiveor exhaustive explanation of the invention. The detailed description isincluded to provide further information about the present patentapplication.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings, which are not necessarily drawn to scale, illustrategenerally, by way of example, but not by way of limitation, variousembodiments described in the present document.

FIG. 1A shows a schematic representation of a battery layer showing anaggregate of particles on a lead substrate, with arrows indicating theflow of ions, according to an example.

FIG. 1B shows a schematic representation of a simplified representationof the layer of FIG. 1A, according to an example.

FIG. 2A shows a pore with low current density, according to an example.

FIG. 2B shows a pore with high current density, according to an example.

FIG. 3 shows three layers with thinner active mass replacing a singlelayer, according to an example.

FIG. 4 shows a stacked or bipolar battery configuration, includingalternating layers and spacers soaked with electrolyte, according to anexample.

FIG. 5 shows a method for assembling the stacked battery. Battery layersand spacers are alternately stacked. The stack is placed in a frame. Thegap between the stack and frame is filled with adhesive. After theadhesive sets, electrolyte is added (absorbed by the spacers) and a capis placed on the top, according to an example.

FIG. 6 shows a method for assembling the stacked battery using removablespacers, according to an example.

FIG. 7A shows a top view of a plate-shaped spacer including taperededges, according to an example.

FIG. 7B shows a front view of a U-shaped spacer, according to anexample.

FIG. 7C shows a fiberglass spacer having an edge lining of a fiberglassspacers, according to an example.

FIG. 8 shows a flow chart of a battery assembly process, according to anexample.

FIG. 9 is a cross-sectional view of a battery layer with an active masscoating showing different layers, from left to right showing silicon,nickel silicide, barrier layer, lead oxide, according to an example.

FIG. 10 is a flow chart of the process for making the battery layer,including formation of a silicide contact and addition of layers to forma barrier to protect the layer from acid corrosion and to promoteadhesion of the active mass, according to an example.

FIG. 11A shows a mix of particles and a matrix, according to an example.

FIG. 11B shows the matrix once the particles are removed, according toan example.

FIG. 11C shows the matrix with plating, according to an example.

FIG. 11D shows the matrix removed, according to an example.

FIG. 12 is a flow chart for formation of a porous active mass, accordingto an example.

FIG. 13 is a micrograph of a wax matrix with pores from dissolved saltparticles, according to an example.

FIG. 14 is a chart showing weight distribution in a conventionallead-acid battery.

DETAILED DESCRIPTION

Examples described herein can retain the low cost and market acceptanceof lead-acid batteries while improving their performance such as to meetthe needs of the electric vehicle and renewable energy markets. Theseexamples can take advantage of the acceptance and maturity of thelead-acid battery and its infrastructure, providing a solution that isfamiliar to risk-averse markets. Many of the present examples may alsobe used to simplify manufacture or design of other types of batteries.

To frame the contributions of the present subject matter, it is helpfulto consider attributes of conventional lead-acid batteries. Conventionallead-acid batteries have a number of limitations. First, conventionallead-acid batteries should run at low current for high efficiency incharging and discharging. This is because a reaction product, leadsulfate, can build up and block electrolyte diffusion, making activemass material (a.k.a. active material) located deep in the batterystructure (referenced in the discussion of FIGS. 2A-B below)inaccessible to chemical reaction. This effect is known as Peukert'sLaw, which represents how battery capacity decreases as charging ordischarging current increases. Due in part to this phenomenon,conventional lead-acid batteries should be charged or discharged over along time, e.g. tens of hours, to show improved efficiency.Unfortunately, most renewable energy storage and vehicle applicationsdesire much shorter discharge times, e.g., from 2 to 6 hours.

Second, conventional lead-acid batteries can demonstrate a reduced lifewhen cycled at deep discharge. Active mass can expand 20-60% in volumeas it converts from lead or lead oxide to lead sulfate. This expansioncreates stress and can cause delamination of a pasted active mass (thatis, active mass applied as a paste, which is a conventional commercialprocess). Because of this, conventional lead-acid batteries should berun in shallow discharge of from 40 to 60%. This can increase the numberof batteries needed for some applications, doubling it in someinstances.

Third, high lead content can result in low energy density. Lead, whichis resistant to the sulfuric acid electrolyte, is used in conventionalbatteries as active mass, as well as being used in terminals or topleads and to provide thick internal conductors to interconnect layers.Typical specific energies for lead batteries can be from 40 to 45W·hr/kg, vs. a USABC target of 100 W·hr/kg. FIG. 14 shows the weightdistribution in a conventional lead-acid batteries used for traction.The subject matter described here can eliminate or greatly reduce thenegative active mass, positive and negative grid, and top leadcomponents, removing about half the lead found in a conventionallead-acid battery. The subject matter described has the potential toeliminate weight (around half in some examples) and can increase(doubling in some examples) the energy density.

Fourth, conventional lead-acid batteries can be low voltage, highcurrent devices. These properties are a poor match to higher voltagesystems used in vehicles and renewable energy systems.

Attempts to overcome limitations of conventional lead-acid batterieshave been met with obstacles. Low efficiency at high current affectsbatteries made with the conventional approach of using active materialapplied as a paste. This mature and low cost approach continues to beused in contemporary lead-acid batteries designs, including high-endbatteries.

Efforts have been made to improve cycle life. One approach is to replacethe negative active mass (“NAM”) with a carbon electrode. Hydrogen canintercalate in the carbon in a manner similar to lithium intercalationin a lithium-ion battery. This can reduce or eliminate shedding on thatlayer. Another approach can integrate a super-capacitor with aconventional battery to provide extended life for repeated power burstsneeded for start-stop cycles.

Lead content has been improved (i.e., decreased) and voltage has beenimproved (e.g., increased) with a bipolar lead-acid battery (such asBlead-acid batteries, or bipolar batteries). Examples can include aseries-connected stack of cells, operating at high voltage and lowcurrent. This configuration can reduce or eliminate heavy internalconductors used in low voltage, high current batteries, and can providea high voltage output.

Blead-acid batteries promises advantages such as high energy density byvirtue of reduced conductor mass. However, several issues have limitedcommercialization. These include cell-to-cell leakage, layer degradationin a corrosive environment that includes both the sulfuric acidelectrolyte and oxygen radicals formed during charging, active massshedding, and electrode sagging that presents issues for layerseparation.

An approach to address the layer degradation issue can use ceramicconducting TiO₂ substrates. The active material is a paste as inconventional lead-acid batteries. Ceramic layers can be less susceptibleto sagging, but may be hard to manufacture in high volume at low cost.

However, even these approaches have shortcomings Typical Blead-acidbatteries designs do not address Puekert's Law limitations. The sourceof these limitations can be understood with the help of FIGS. 1A, 1B, 2Aand 2B.

FIG. 1A shows a schematic representation of a battery layer showing anaggregate of particles on a lead substrate, with arrows 110 indicatingthe flow of ions, according to an example. The pasted active mass layer102 can include an aggregate of particles 104 disposed on a substrate108, which can be a few microns in diameter. Electrolyte can flowthrough channels between the particles. The channel diameter can be afew microns and the length can be substantially similar to the thicknessof the active mass, 1-3 millimeters in some examples.

FIG. 1B shows a schematic representation of a simplified representationof the layer of FIG. 1A, according to an example. As depicted in FIG.1B, channels 106 can be theoretically modeled as straight channels. Atlow currents, the electrolyte ions can diffuse the length of the channelwith low, or even without, depletion, and the reaction can proceed alongthe full length of the channel 106. At high currents, the electrolyteions can be consumed before they can diffuse the full length of thechannel 106. As a consequence, at high currents the active mass deep inthe layer does not react as desired, and the available energy, which isassociated with available reactions with the AM, can decrease.

FIG. 2A shows a pore with low current density, according to an example.FIG. 2B shows a pore with high current density, according to an example.In these examples, channel 206, which represent pores 208, have a leadsulfate coating 210. The current at onset of ion depletion scales as1/L², where L is the channel length. The narrowing 212 represents thatmore reactions have taken place than at less narrow portions.

FIG. 3 shows three layers with thinner active mass replacing a singlelayer, according to an example. One solution to the problem ofunbalanced ion depletion is to split the active mass into severalthinner layers, as shown in FIG. 3, in which the length “L” shown inFIG. 2 has been reduced by a factor of 1/X to provide a shorter channel302. To compensate for the reduction in total channel length by the 1/Xreduction, more channels 302′, 302″ can be used. They may total X innumber, but other numbers are possible. Such a configuration can retainthe same amount of active mass, so the battery can retain the same or asimilar capacity. Because of the shorter channels, the battery can runat higher current while accessing a greater portion of the active mass.For example, dividing a single 1 millimeters thick active mass layerinto three 0.3 millimeters thick active mass layers provides 9 timesmore current without loss of capacity.

This approach and others disclosed herein can be used to overcomeshortcomings described above. New systems and methods described hereinprovide battery layers with thin active mass layers. These layers can beclosely spaced, and the amount of active mass can remain constant toretain a desired battery capacity. Additionally, these thin active masslayers have other desirable attributes.

For example, a lead layer expands about 60% when converted to leadsulfate, and the lead oxide layer expands about 20%. This expansion cancause shedding of the active mass in deep cycling. A thinner layer hasless mechanical stress at the interface, and is less likely to shed,allowing the battery to operate reliably in deep cycling.

Examples disclosed herein provide a bipolar lead-acid battery withlayers that can be much thinner than conventional plates, which canenable balanced ion depletion. Silicon wafers can be used as substratesand provide layers that are light, resistant to reaction with sulfuricacid, and that are inexpensive. Active mass layers can be formed usingplating or electrophoretic deposition instead of pasting, enablingcontrolled formation of thin layers. The composition can be varied indepth to provide for selected critical properties such as porosity,grain size, and stress. Contact and barrier layers can be included. Asacrificial template process is described by way of example to providecontrolled porosity, employing one or both of deposition of asacrificial layer and co-deposition using electrophoresis. Methods topackage the battery are also described, and can include sealing a stackof layers in a molded form, adding electrolyte, and affixing a cover.

Examples provide a bipolar lead-acid battery design that enables the useof thin layers to provide a battery with an increased layer density overthat of conventional batteries. Examples allow spreading the active massover a large number of thin layers to reduce the effect of Puekert'sLaw, enabling deep cycling (i.e., balanced ion depletion) with reducedshedding of active mass. Examples provide a high voltage output suitablefor electric vehicle and renewable energy systems. Examples use less(half in some examples) of the lead of conventional lead-acid batteries,which can increase (double in some examples) energy and power density.

FIG. 4 shows a stacked or bipolar battery configuration, includingalternating plates and separators or spacers soaked with electrolyte,according to an example. A electrochemical battery has two terminals: acathode (positive) and anode (negative). A reduction reaction occurs atthe cathode and an oxidation reaction occurs at the anode. The batterypotential is the sum of the half-reaction voltages. In the case oflead-acid, the positive plate is typically lead oxide, and thehalf-reaction voltage is about 1.6 volts. The negative plate istypically lead, and the half-reaction voltage is 0.4 volts.

A battery cell includes, at a minimum of an anode and cathode. Voltagesfor cells wired in series are additive. Accordingly, 10 lead-acid cellsconnected in series can provide 20 volts (e.g., 10 cells×2 volts/cell).In an example series connection, the string can include a series ofanodes connected to cathodes, with the intervening electrolyteselectrically isolated.

The example includes a stack 400 of layers such as plates 408 packagedwith spacers 406. The stack 400 can include one or more anodes 410separated from cathodes 412, such as by spacers or separator 414. Gapsbetween the plates can be filled with sulfuric acid electrolyte. Theelectrolyte masses or volumes can be electrically isolated so that theplates can be in series. The spacer material can be fiberglass, which isporous and can absorb sufficient sulfuric acid. The plate spacing can be0.5 millimeters.

Separators or spacers 408 can prevent shorting of the plates, and can bethin sheets of fiberglass. In some cases, the plates are stiff, and insome of those examples spacers are not necessarily used. Electrolyte,which can be sulfuric acid, can be disposed in a space between plates.Electrolyte can be soaked into the spacers.

If multiple electrolyte masses or volumes can be electrically isolatedfrom one another, and there is a conduction path from the anode to thecathode, such as through the use of plates that are conductive, thestack can form a series-connected arrangement of cells. Voltage can beequal to (N−1)V_(Cell), where N is the number of plates (with one ateach end for connection to the positive 404 and negative 402 terminals),and V_(Cell) is the voltage of a single cell.

For example, the cell voltage for the lead-acid reaction can be around 2volts. Accordingly, a battery having 101 plates can have a voltage of200 volts. A housing 406 is shown, mechanically maintaining multiplecells in a stack.

Examples can include electrically conducting substrates with an anode onone side and cathode on the other. The substrate can act as theconductor or “wire” to connect the cells together while isolating theelectrolytes from one-another. In some examples it is possible toeliminate the lead electrode entirely by using a carbon or siliconcounter-electrode. This can provide even higher energy density. Suchexamples can use bare silicon or carbon coated silicon as the counterelectrode to the lead oxide electrode.

Note that the cathode can provide most of the cell voltage. Some casesomit lead as the material for the half cell reaction at the anode whileproviding a place for a reduction reaction to occur on that side of thecell. One way to accomplish this is to allow protons from theelectrolyte solution to react (intercalate), as occurs in other types ofbatteries such as lithium ion. This reaction operates in both carbon andsilicon. In such a case, the cell voltage can be at least 1.6 volts (thecathode half-cell potential), but the mass and weight of the lead on theanode can be reduced or eliminated, resulting in an increase in powerand energy density and reductions in cost and toxic material content.Life can also be extended because lead suffers the greatest expansionwhen it converts to lead sulfate, and therefore undergoes the greateststress. The loss of voltage can easily be made up by adding moreseries-connected cells.

An exposed leftmost electrode 402 can serve as a terminal, such as forcoupling electrically and mechanically with electronics. The exposedmajor face of the rightmost terminal 404 can serve as an electrode ofthe opposite polarity, and can serves as a terminal as well. However, insome examples, the stack is disposed in a housing or container and isconnected to electronics outside the housing via one or morefeedthroughs extending through the housing.

The anode is shown having a plurality of protrusions 416 definingchannels 418. However, the present subject matter is not so limited, andexamples in which the cathode has protrusions are also contemplated, asare examples in which no protrusions are used.

An unexpected result is that such a battery can in some examples, usesilicon wafers with standard solar cell texture. Some examples usetextured silicon, such as cut wafers. As-cut silicon wafers, originallyused for solar cells, can be used as the substrates for the electrodes.These wafers are light (about a quarter the density of lead), can beresistant to sulfuric acid corrosion, and can be generally available atlow cost by virtue of their high volume of use. As-cut wafers can have asurface roughness that provides good adhesion, such as for mechanicallyjoining with a coating. For example, multi-crystal (MC) wafers can beformed by iso-texturing, such as in a bath of hydrofluoric acid andnitric acid.

Multi-crystalline wafers can provide a square form factor and lowercost. Single crystal wafers can also be used. Single crystals can have apyramidal texture, typically formed with a potassium hydroxide(“KOH”)/isopropyl alcohol etch. Because large grain size is not asimportant, MC wafers can be made more rapidly than they are for solarapplications, which can provide for lower cost. A lower costmetallurgical grade silicon can be used, as its purity is compatiblewith battery applications disclosed herein. Other silicon, such aselectronic, solar or semiconductor grade can be used, but are generallymore expensive.

In some examples, wafers can be doped. Doped wafers can have aresistivity typically less than 1 Ω-cm. In some examples, theresistivity can be less than 0.001 Ω-cm. Lower resistivity can improveefficiency as battery current flows through the wafers. Low resistivitycan also improve the quality of contacts to the silicon. Dopants can beused, such as phosphorus, boron, antimony or arsenic. Such wafers can beless than 500 μm (0.5 millimeters) thick, and can be less than 200 μmthick.

Wafers can be square, with an edge length of 156 millimeters forstandard solar cell wafers, although rectangular wafers, or wafers withother form factors such as clipped corners can also be used. Use of astandard edge length can enable the use of wafers manufactured in highvolume, which can reduce cost, although other edge lengths can be used.Use of standard size wafers can allow for the use of standardmanufacturing equipment to handle and process the wafers during batterymanufacturing.

In certain examples, active mass can be formed on one or both sides of asubstrate. Lead can be plated onto both sides. One plated side can bemasked and the other can be exposed to a sulfuric acid bath. Whileexposed, a current can be run through such a bath using a lead negativeelectrode. Such an approach can convert the exposed side to lead oxideusing a process termed “forming.”

In certain examples, only one side of the silicon substrate is coatedwith lead and converted to lead oxide or, alternately, coated with leadoxide. In one-sided examples, the battery can have a lower voltage thatan example with active material lead coated on both sides. In someexamples, the half-cell potential for lead oxide to lead sulfatereaction can be 1.68 volts. A battery with lead coated on only one sidecan use less (e.g., half) lead, so it can be less toxic and lighter inweight. In addition, lead can expand more than lead oxide when convertedto lead sulfate, so plates without a lead coated side can experienceless stress during cycling.

According to various examples, either one or both sides can be coatedwith active mass. Other materials can be used as active mass and the useof the silicon plates is not exclusive to lead-acid type batteries. Insome instances configured as single-sided, hydrogen can intercalate intothe silicon on the opposite electrode, much as lithium does in alithium-ion battery. Note that a silicon surface can be coated with aninert material such as carbon, and hydrogen can intercalate into thecarbon. Such intercalation can be beneficial, as it can help a cellresist bulging due to gas production.

FIG. 5 shows a method for assembling the stacked battery. On the left, aside view of battery plates and spacers alternately stacked is shown. Asillustrated, alternating layers of plates 502 and fiberglass spacers 504can be stacked, as shown in the left drawing in FIG. 5. Note that therecan be a plate at each end to form the positive and negative terminalsor poles of the battery. The battery stack can be placed in a U-shapedframe 506 that provides three sides. An adhesive that is resistant orimpervious to sulfuric acid, such as epoxy or any of a number ofplastics resistant to sulfuric acid, such as polypropylene, can beinjected into the space 508 between the u-shaped frame 506 and thebattery stack 500. After the adhesive has set electrolyte can be addedand a cover 510 can be put in place. In some examples, fiberglassspacers can resist or prevent adhesive from seeping into the spacebetween the plates any more than a small region near the edges of thefaces of the plates. It can be helpful to seal the edges 512 of thestack so that the electrolyte masses (i.e., volumes with electrodes ofopposite polarity on opposing sides) can be electrically isolated. Insome cases, laser cut grooves can be formed near the periphery of theplates, using laser grooving equipment common in solar cellmanufacturing. Such grooves can be 10-20 μm deep and on the order of 50μm wide. This can provide a re-entrant structure to improve the qualityof the edge seal.

FIG. 6 shows a method for assembling the stacked battery using removablespacers, according to an example. In some cases it is desirable to haveadditional space in the gap 604 between the plates, thereby providingroom for extra electrolyte. One example providing this space is the useof removable spacers 602, as shown in FIG. 6. A battery stack can bemade with spacers 602 that extend out of the stack on the top side, asshown in the side view on the left. The battery stack can be placed in aU-shaped frame 606, shown in front view in the center, that providesthree sides. An adhesive that is resistant or impervious to sulfuricacid, such as epoxy or any of a number of plastics resistant to sulfuricacid, can be injected into the space 608 between the U-shaped frame 606and the battery stack 600. After the adhesive or plastic sets, thespacers can be pulled out, electrolyte can be added, and a cover 610 canbe put in place. The cover 610 can have a vent to prevent gas pressurebuild-up if the battery is overcharged, and can be removable to allowrecharging of electrolyte.

FIG. 7A shows a top view of a plate-shaped spacer including taperededges, according to an example. The edges of the spacers can be tapered,as shown in FIG. 7A, which is a top view. The tapered edges 706extending away from a main body 708 can reduce contact area between thespacer an another structure such as a frame, providing for easierremoval. They also provide a tapered region that can be filled with theepoxy or plastic to provide an improved seal.

FIG. 7B shows a front view of a U-shaped spacer, according to anexample. As illustrated in FIG. 7B, spacers can be U-shaped, with aspacer portion 710 defining an inner void 712. Such a spacer can allowfor removal by pinching the ends 714 in the direction of the arrows andlifting the spacer out of a frame, a process that can permit air toenter the frame to ease spacer removal.

FIG. 7C shows a fiberglass spacer having an edge lining of a fiberglassspacers, according to an example. In some cases, fiberglass separatorscan wick the glue so that it extends excessively into the space betweenthe plates. Forming an edge liner 704 around the fiberglass spacers 702,as shown in FIG. 7C, can prevent this. In some examples, the fiberglasscan be melted to form a glass frame that does not wick adhesive. In someexamples, adhesive or plastic can be applied to the rim of the spacersto form a frame consisting of set adhesive. In some examples, the edgeliner can be soft and flexible if an appropriate adhesive such assilicone is used.

The spacers can be made of a non-stick material such as Teflon, or canhave a Teflon coating to ease removal. They can also have holes throughthe top that can be aligned so that one or more rods can be passedthrough the set of spacers, simplifying alignment and removal. A moldrelease material can be applied to one or more surfaces to provide foreasier removal.

In some examples, the edges of the silicon plates may have nicks ordefects resulting during their manufacture. These nicks can cause theplates to break when handled. The plates can be coated with epoxy orplastic before assembly. This is called pre-coating. It can protect theedges, to reduce the risk of breaking wafers. The coating can be bydipping or direct application. In some examples, the coating thicknessequals half the plate separation. In some examples, the plates can bestacked and an additional layer of epoxy or plastic applied to form theouter housing of the battery. The pre-coating material can be a plasticsubstantially resistant to sulfuric acid. The sum of the thicknesses ofthe pre-coating on plate faces can be approximately equal a plateseparation between plates.

FIG. 8 shows a flow chart of a battery assembly process, according to anexample. The process can be used to produce the examples discussed inFIGS. 4-6 and other disclosed herein. At 802, electrodes are placed intoa stack. At 804, the electrode stack is placed in a frame. At 806,adhesive is added to adhere the stack to the frame. At 810, an optionalstep allows for removal of at least some spacers. At 812, one or moreinterior space defined between electrodes adhered to the frame can befilled with electrolyte. At 814, a cover or top can be added to theframe to seal in the electrolyte.

FIG. 9 is a cross-sectional view of a battery plate with an active masscoating showing different layers, from left to right showing silicon,nickel silicide, barrier layer, lead oxide, according to an example.FIG. 9 shows an example multiple layer stack. The figure shows a plate900 comprising silicon 902, nickel silicide 904, a barrier layer 906 andlead oxide 908. It should be noted that it is often desirable to removeany native oxide from the silicon before applying a layer. This can bedone with sandblasting or using a chemical etch such as bufferedhydrofluoric acid.

It can be beneficial to form layers between the silicon substrate andthe active mass. One benefit is to improve contact between the substrate(e.g., silicon) and the active mass. Some examples interpose a silicidelayer between the substrate and the active mass. Some examples interposea nickel silicide layer between the substrate and the active mass. Sucha layer can be formed using an electroless nickel deposition or a vacuumprocess such as evaporation or sputter deposition. Some examples includea heating cycle such as at 500° C. Some examples heat for around 10seconds. A silicide layer can be formed on the opposite side to improvecontact to the inert layer (e.g., carbon) or to the electrolyte. In someexamples, other silicides such as molybdenum, titanium, tungsten andtheir alloys can be used instead of or in addition to nickel.

Additional layers can be added for protecting the silicon from reactionwith the electrolyte and to improve adhesion of the active mass to thesubstrate. Such layers can include TiN, TaN, molybdenum selenide, tin orchrome, and can be formed on one or both sides of the substrate. Methodsof deposition include, but are not limited to, sputtering, reactivesputtering or evaporation. Barrier or adhesion layers can be relativelythin, such as from 20 to 100 nm.

FIG. 10 is a flow chart of the process for making the battery plate,including formation of a silicide contact and addition of layers to forma barrier to protect the plate from acid corrosion and to promoteadhesion of the active mass, according to an example. The process can beused on one or both faces of a layer. At 1002, the method starts byproviding a silicon substrate. At 1004, the substrate can be cleaned toremove contamination and organic deposits. Cleaning solutions that canbe used include a mixture of sulfuric acid and hydrogen peroxide toremove organics. The surface can also be etched in hydrofluoric acid toremove any oxide layer that forms after the sulfuric/peroxide clean, orcan be sandblasted. At 1006, electroless nickel can be deposited.Optionally, electroless nickel can be can be vacuum deposited asdescribed above. At 1008, the deposit can be baked. Such a deposit canbe heated at 300-700° C., such as for 30 seconds to form a silicidecontact layer. At 1010, a barrier can be deposited. The barrier can beplated or sputtered, among other methods of forming. At 1012, anadhesion and/or barrier layers can be deposited. At 1014, the activemass can be formed using methods described herein. At 1016, the activemass can be conditioned, for example, to turn it from lead to leadoxide. In some examples, lead can be plated directly to the silicon. Thelead can optionally be heated at 200° C. for 5 minutes to improvecontact and adhesion.

Examples can form an active mass with controlled porosity and pore size.In some examples, the active mass can be plated. The active mass can beless than 1 millimeter thick. Some examples are from 0.2 to 0.3millimeters thick.

In some examples, the active mass can include lead(IV) oxide, PbO₂. Thenotation lead “(IV)” refers to lead with a valence of +4. A platedmaterial can also include lead, which can be electrolytically convertedto PbO₂ using forming or conditioning. In some conditioning processes,current can be run through the plate in a 6 molar sulfuric acid bath toconvert it to lead sulfate. The current can be reversed to form leadoxide on a positive plate.

FIGS. 11A-C show a pictorial representation of the process of making aporous active mass, according to an example. A deposition can be madeporous using various methods. In some examples, additives can be put inthe plating solution, such as those used to make a matte finish plating.In some examples, a sacrificial layer 1102 can be used. A mix of finesoluble particles 1104 and a matrix material 1106 such as a cured resinsuch as paraffin wax or a polymer such as etch-resist can be prepared.The particles 1104 can have the same size as the active mass grains,which can be around 5 μm diameter. They can be of a soluble materialsuch as a crystalline salt, sodium chloride being one example. The mixcan be applied to the substrate 1108, which can be heated to allow thematrix (e.g., paraffin) to flow. The mix 1102 can be allowed tosolidify, by, for example, cooling or evaporation of organicconstituents. The wafer can be placed in water so that the solubleparticles 1104 dissolve. Such a process can produce a porous organicmatrix 1106.

Once the porous matrix is created, the wafer can be placed in a platingbath. The active mass material can be plated into the pores. The matrix1106 can be thicker than the plating 1110 thickness, which can bedetermined by the plating time and current. The matrix can be dissolvedin a solvent to leave the porous active mass layer, which can beconditioned to form lead(IV) oxide if the original plating material waslead.

In some examples, electrophoretic deposition can be used to deposit theactive mass. Electrophoresis is a process in which charged particles canbe attracted to an electrode. In an example process a suspension ofactive mass particles can be made in an ethanol bath, such as by usingultrasonic agitation. One benefit of ethanol, and compositions thereof,is that it is a poor conductor of electricity, so a field can beestablished across the bath. A small amount of sulfuric acid can beadded to the suspension, for example 0.5 milliliters per 100 millilitersof bath. Such an addition can provide a source of ions to chargesuspended active mass particles. The electrode to be coated can beplaced in the bath and connected to the negative terminal of a voltagesource, such as a 50-200 volt source, with an electrode spacing on theorder of 2-5 centimeters. The potential urges active mass particles tothe surface, where they deposit. A coated plate can be baked at atemperature exceeding 100° C. Some examples are baked at 200° C. for 30minutes. Baking can drive ethanol out of the coating.

Other materials can be co-deposited with this method, including, but notlimited to, fiber and chemical binders. Such materials can improveadhesion and the integrity of the film. Integrity as used herein refersto resistance to flaking or decomposition of the active mass layer. Asoluble species such as salt grains can also be co-deposited, and can bedissolved as described above to control film porosity. This has theadvantage of reducing or eliminating the need for sacrificial paraffinand subsequent plating steps.

FIG. 12 is a flow chart for making a porous active mass, according to anexample. The flowchart can be used to make the apparatus of FIG. 11. At1202, a filled matrix can be applied to a substrate. At 1204, filler inthe matrix can be dissolved. At 1206, the matrix can be plated. At 1208the matrix can be dissolved. At 1210 the remaining material can beconditioned.

FIG. 13 shows a micrograph 1300 of a paraffin matrix 1302 with holes1304 left behind after salt crystals have been dissolved. The ratio ofmatrix material to particles can determine the porosity. Particle sizeand shape can determine the pore size. The mixture can contains 50-70%solids. A high solid fraction can encourage the formation of pores thatare continuous, which enables thorough plating throughout the matrix. Insome cases matrix material wets the top surface, in which can thesurface can be lightly scraped to expose salt.

The consistency of the active mass can be varied in depth. For example,multiple sequential depositions can be layered on top of one another.During a deposition, parameters can be altered, making it possible tovary parameters such as grain size, porosity, composition, or filmstress.

Various Notes & Examples

Example 1 can include or use subject matter (such as an apparatus, amethod, a means for performing acts, or a device readable mediumincluding instructions that, when performed by the device, can cause thedevice to perform acts), such as a stack of electrodes, including: afirst electrode including a silicon substrate and an active material oractive mass disposed on the silicon substrate, a second electrodedisposed in the stack in alignment with the first electrode, and aseparator disposed between the first electrode and the second electrode.The example can include a housing, with the stack of electrodes disposedin the housing, electrolyte filling the housing and in contact with thefirst electrode and the second electrode, a seal coupled between thehousing and the stack to define an interior space extending between thefirst electrode and the second electrode, the seal adapted to resist theflow of electrolyte from the interior space, a cover coupled to thehousing, and a cover seal adapted to resist the flow of the electrolytefrom inside the interior space.

Example 2 can optionally can optionally include the subject matter ofany of the preceding examples 1, wherein a major face of the firstelectrode is exposed to an exterior, the second electrode is of adifferent polarity, and a second major face of the second electrode isexposed to an exterior, opposite the first major face.

Example 3 can optionally include the subject matter of any of thepreceding examples, wherein the active material includes lead (or a leadcompound) and the electrolyte includes sulfuric acid.

Example 4 can optionally include the subject matter of any of thepreceding examples in which at least one intervening layer is disposedbetween the substrate and the active material.

Example 5 can optionally include the subject matter of any of thepreceding examples wherein the intervening layer is formed of at leastone of a group including TiN, TaN, molybdenum selenide, tin and chrome.

Example 6 can optionally include the subject matter of any of thepreceding examples wherein the intervening layer includes a silicide.

Example 7 can optionally include the subject matter of any of thepreceding examples wherein the silicide includes tungsten, titanium,nickel or molybdenum.

Example 8 can optionally include the subject matter of any of thepreceding examples, wherein the substrate is less than 0.5 millimetersthick and the active material is less than 0.5 millimeters thick.

Example 9 can optionally include the subject matter of any of thepreceding examples, wherein the substrate has a cut surface onto whichthe active material is disposed.

Example 10 can optionally include the subject matter of any of thepreceding examples, wherein the active material is porous.

Example 11 can optionally include the subject matter of any of thepreceding examples, wherein a major face of the substrate has arectangular perimeter, with side lengths of approximately 156millimeters.

Example 12 can optionally include the subject matter of any of thepreceding examples, including forming a battery electrode by disposingan active material coating onto a silicon substrate, assembling thebattery electrode into a stack of battery electrodes, the batteryelectrode separated from other battery electrodes by a separator,disposing the stack in a housing, filling the interior space withelectrolyte, and sealing the housing to resist the flow of electrolytefrom the interior space.

Example 13 can optionally include the subject matter of any of thepreceding examples in which the plated coating is lead, and furtherincluding oxidizing the coating after application to form lead(IV)oxide.

Example 14 can optionally include the subject matter of any of thepreceding examples, including forming a silicide between the substrateand the active material by disposing a nickel onto the substrate andheating the substrate.

Example 15 can optionally include the subject matter of any of thepreceding examples wherein disposing the nickel includes plating thenickel including applying the nickel using electroless deposition.

Example 16 can optionally include the subject matter of any of thepreceding examples in which the silicide is formed by sputtering orevaporating a metal and heating the substrate.

Example 17 can optionally include the subject matter of any of thepreceding examples, wherein the active material is a porous platedactive material formed by: disposing a sacrificial layer of matrixmaterial and particles onto the substrate, dissolving the particles toform a matrix with pores, plating active material into at least somepores, and dissolving the matrix.

Example 18 can optionally include the subject matter of any of thepreceding examples, wherein disposing the active material onto thesubstrate includes applying the substrate using electrophoresis.

Example 19 can optionally include the subject matter of any of thepreceding examples, including mechanically fixing the stack to thehousing to define an interior space, with the separator disposed in theinterior space, and removing the separator.

Example 20 can optionally include the subject matter of any of thepreceding examples, wherein disposing the active material includeselectrophoretic co-deposition of the active material along with asacrificial material, and defining a porous active material bydissolving the sacrificial material after electrophoretic co-deposition.

Example 21 can optionally include the subject matter of any of thepreceding examples, in which the silicon substrate is highly doped.

Example 22 can optionally include the subject matter of any of thepreceding examples in which the silicon resistivity is less than 1 Ω-cm.

Example 23 can optionally include the subject matter of any of thepreceding examples which the substrate has an as-cut surface.

Example 24 can optionally include the subject matter of any of thepreceding examples which the silicon has a standard solar cell texture.

Example 25 can optionally include the subject matter of any of thepreceding examples which the silicon is metallurgical grade material.

Example 26 can optionally include the subject matter of any of thepreceding examples which the substrate is multi-crystal silicon.

Example 27 can optionally include the subject matter of any of thepreceding examples including a process for applying an active masscoating to a silicon battery plate in which the coating is plated ordeposited using electrophoresis.

Example 28 can optionally include the subject matter of any of thepreceding examples in which the plated or electrophoresis depositedcoating is less than 1 millimeters thick.

Example 29 can optionally include the subject matter of any of thepreceding examples, including intervening layers between the siliconsubstrate and active mass to promote adhesion of the active mass to thebattery plate.

Example 30 can optionally include the subject matter of any of thepreceding examples in which an additive is included in the platingsolution to promote porosity.

Example 31 can optionally include the subject matter of any of thepreceding examples in which a sacrificial layer is applied to thebattery plate, said sacrificial layer consisting of a matrix materialand particles, said particles being subsequently dissolved to form amatrix with pores, at least a portion of said pores being then filled byplating and said matrix being subsequently dissolved, in which thematrix material is at least one of wax and a polymer.

Example 32 can optionally include the subject matter of any of thepreceding examples in which said particles are a crystalline salt.

Example 33 can optionally include the subject matter of any of thepreceding examples in which the crystalline salt is sodium chloride.

Example 34 can optionally include the subject matter of any of thepreceding examples that also includes porous spacers between plates.

Example 35 can optionally include the subject matter of any of thepreceding examples in which the porous spacer material includesfiberglass.

Example 36 can optionally include the subject matter of any of thepreceding examples in which a stack is formed including alternatingbattery plates, porous spacers, and removable spacers, said stack isthen placed in a containment fixture, sealant is applied to theperiphery of said stack, and said removable spacers are removed aftersaid sealant sets.

Example 37 can optionally include the subject matter of any of thepreceding examples in which three sides are sealed, the spacers removed,electrolyte added, and a top cover placed on the battery.

Example 38 can optionally include the subject matter of any of thepreceding examples in which one or more edges of the removable spacersare tapered.

Example 39 can optionally include the subject matter of any of thepreceding examples in which a release coating is applied to theremovable spacers.

Example 40 can optionally include the subject matter of any of thepreceding examples in which the removable spacers have a U-shape so thatremoval includes a step of pinching the ends of the U-shape toward eachother.

Example 41 can optionally include the subject matter of any of thepreceding examples in which the removable spacers are reusable.

Example 42 can optionally include the subject matter of any of thepreceding examples in which a stack is formed including alternatingbattery plates and porous spacers, said stack is then placed in acontainment fixture and sealant is applied to the periphery of saidstack.

Example 43 can optionally include the subject matter of any of thepreceding examples in which three sides are sealed, electrolyte added,and a top cover placed on the battery.

Example 44 can optionally include the subject matter of any of thepreceding examples in which the porous spacers have edge liners toprevent absorption of the sealant into the spacers.

Example 45 can optionally include the subject matter of any of thepreceding examples in which the edge liner is an adhesive.

Example 46 can optionally include the subject matter of any of thepreceding examples in which the adhesive is silicone.

Example 47 can optionally include the subject matter of any of thepreceding examples in which the edge liner is formed by melting the edgeof the fiberglass spacer.

Example 48 can optionally include the subject matter of any of thepreceding examples in which one face of the substrate is inert.

Example 49 can optionally include the subject matter of any of thepreceding examples in which the inert face is silicon

Example 50 can optionally include the subject matter of any of thepreceding examples in which the inert face is coated with carbon.

Example 51 can optionally include the subject matter of any of thepreceding examples in which the inert face is a silicide.

Example 52 can optionally include the subject matter of any of thepreceding examples in which the active mass on at least one side isapplied using electrophoresis.

Example 53 can optionally include the subject matter of any of thepreceding examples in which the active mass material includes leadoxide.

Example 54 can optionally include the subject matter of any of thepreceding examples in which active mass is baked at a temperatureexceeding 100° C. after electrophoretic deposition.

Example 55 can optionally include the subject matter of any of thepreceding examples in which an intervening layer is disposed between theelectrophoretic active mass deposition and the plate substrate.

Example 56 can optionally include the subject matter of any of thepreceding examples in which plate material includes silicon.

Example 57 can optionally include the subject matter of any of thepreceding examples, in which an electrode is formed by electrophoreticco-deposition of an active mass material and a sacrificial material,said sacrificial material being dissolved after electrophoreticco-deposition in order to increase the porosity of the active masslayer.

Example 58 can optionally include the subject matter of any of thepreceding examples active mass layer in which the active mass isco-deposited with a second material, a function of said second materialbeing to improve a physical property of the active mass layer.

Example 59 can optionally include the subject matter of any of thepreceding examples in which the physical property is adhesion.

Example 60 can optionally include the subject matter of any of thepreceding examples in which the physical property is integrity of theactive mass layer.

Example 61 can optionally include the subject matter of any of thepreceding examples including sand blasting the plate surface.

Example 62 can optionally include the subject matter of any of thepreceding examples in which a battery uses a plurality of plates, withat least two plate shaving edges being pre-coated.

Example 63 can optionally include the subject matter of any of thepreceding examples in which the pre-coating material is epoxy.

Example 64 can optionally include the subject matter of any of thepreceding examples in which the pre-coating material is a plasticsubstantially resistant to sulfuric acid.

Example 65 can optionally include the subject matter of any of thepreceding examples in which the sum of the thicknesses of thepre-coating on plate faces approximately equals the plate separation.

Example 66 can optionally include the subject matter of any of thepreceding examples in which a stack of plates is sealed after stackingto provide an outer housing for the battery.

Example 67 can optionally include the subject matter of any of thepreceding examples in which the sealing material is plastic.

Example 68 can optionally include the subject matter of any of thepreceding examples in which the sealing material is epoxy.

Example 69 can optionally include the subject matter of any of thepreceding examples in which the active mass consistency is varied withdepth through the use of either multiple sequential depositions orvarying deposition parameters through the deposition process.

Example 70 can include, or can optionally be combined with any portionor combination of any portions of any one or more of Examples 1-69 toinclude subject matter that can include means for performing any one ormore of the functions of Examples 1-69, or a machine-readable mediumincluding instructions that, when performed by a machine, cause themachine to perform any one or more of the functions of Examples 1-69.

Each of these non-limiting examples can stand on its own, or can becombined in various permutations or combinations with one or more of thesome examples.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect tosome examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable to conFIG.an electronic device to perform methods as described in the aboveexamples. An implementation of such methods can include code, such asmicrocode, assembly language code, a higher-level language code, or thelike. Such code can include computer readable instructions forperforming various methods. The code can form portions of computerprogram products. Further, in certain examples, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) can be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features can be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments can be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. An apparatus, comprising: a stack of battery electrodes, comprising:a first battery electrode comprising a silicon wafer and an activematerial disposed on the silicon wafer; and a second battery electrodedisposed in the stack with the first electrode, physically spaced apartfrom the first electrode, and a housing, with the stack of batteryelectrodes disposed in the housing; and electrolyte disposed in thehousing and in contact with the first electrode and the secondelectrode.
 2. The apparatus of claim 1, wherein a first face of thefirst electrode is exposed to an exterior of the apparatus, the secondelectrode is of a different polarity, and a second face of the secondelectrode is exposed to an exterior of the apparatus, opposite the firstface.
 3. The apparatus of claim 1, wherein the active material includeslead, the electrolyte includes sulfuric acid, the silicon wafer isformed of metallurgical grade silicon wafer of a resistivity less thanor equal to 1 Ω-centimeter, and the electrodes are arranged in a bipolarconfiguration, and further comprising: a seal coupled between thehousing and the stack to define an interior space extending between thefirst electrode and the second electrode, the seal adapted to resist aflow of electrolyte from the interior space; a cover coupled to thehousing; and a cover seal between the cover and the housing, the coverseal adapted to resist the flow of the electrolyte from inside theinterior space.
 4. The apparatus of claim 1 in which at least oneintervening layer is disposed between the wafer and the active materialwherein the at least one intervening layer is formed of at least one ofa group including TiN, TaN, molybdenum selenide, tin and chrome and asilicide, wherein the silicide includes tungsten, titanium, nickel ormolybdenum.
 5. The apparatus of claim 1, wherein the silicon wafer isless than 0.5 millimeters thick and the active material is less than 0.5millimeters thick, and the silicon wafer has a resistivity less than orequal to 1 Ω-cm.
 6. The apparatus of claim 1, wherein the wafer has acut surface onto which the active material is disposed and the activematerial is porous.
 7. A method, comprising: forming a battery electrodeby disposing an active material coating onto a silicon wafer; assemblingthe battery electrode into a stack of battery electrodes to define abattery cell, the battery electrode separated from at least one otherbattery electrode by a separator to define an interior space between theelectrode and the at least one other battery electrode, the interiorspace being one of a plurality of interior spaces in the stack that areelectrically isolated from one another; disposing the stack in a housingsuch that electrodes define an interior space between them; filling theinterior space with electrolyte; and sealing the housing to resist aflow of electrolyte from the interior space.
 8. The method of claim 7 inwhich the coating is further oxidized after application to form lead(IV)oxide.
 9. The method of claim 7, comprising forming a silicide betweenthe wafer and the active material by disposing nickel onto the wafer andheating the wafer.
 10. The method of claim 9 wherein disposing thenickel includes plating the nickel including applying the nickel usingelectroless deposition.
 11. The method of claim 9 in which the silicideis formed by one of sputtering and evaporating a metal, and heating thewafer.
 12. The method of claim 7, wherein the active material is aporous plated active material formed by: disposing a sacrificial layerof matrix material and particles onto the wafer; dissolving theparticles to form a matrix with pores; plating active material into atleast some pores; and dissolving the matrix.
 13. The method of claim 7,wherein disposing the active material onto the wafer includes one ofapplying the active material using electrophoresis, electrophoreticco-deposition of the active material along with a sacrificial materialand electrophoretic co-deposition of the active material along with asecond material.
 14. The method of claim 7, comprising mechanicallyfixing the stack to the housing to define an interior space, with theseparator disposed in the interior space, and removing the separator.15. The method of claim 7, further comprising forming pores in theactive material coating by dissolving a sacrificial material afterelectrophoretic co-deposition.
 16. An apparatus, comprising: a stack ofbattery electrodes, comprising: a first battery electrode comprising asilicon wafer and an active material including lead disposed on thesilicon wafer; and a second battery electrode disposed in the stack withthe first electrode, physically spaced apart from the first electrode,and a housing, with the stack of battery electrodes disposed in thehousing; and electrolyte disposed in the housing and in contact with thefirst electrode and the second electrode.
 17. The apparatus of claim 16,wherein a first face of the first electrode is exposed to an exterior ofthe apparatus, the second electrode is of a different polarity, and asecond face of the second electrode is exposed to an exterior of theapparatus, opposite the first face.
 18. The apparatus of claim 16 inwhich at least one intervening layer is disposed between the wafer andthe active material wherein the at least one intervening layer is formedof at least one of a group including TiN, TaN, molybdenum selenide, tinand chrome and a silicide, wherein the silicide includes tungsten,titanium, nickel or molybdenum.
 19. The apparatus of claim 16, whereinthe silicon wafer is less than 0.5 millimeters thick and the activematerial is less than 0.5 millimeters thick, and the silicon wafer has aresistivity less than or equal to 1 Ω-cm.
 20. The apparatus of claim 16,wherein the wafer has a cut surface onto which the active material isdisposed and the active material is porous.